Seal for a sensor element of a gas sensor

ABSTRACT

A seal for a sensor element for a gas sensor for determining the oxygen content in a gas to be measured including exhaust gases of internal combustion engines, the seal including a metallic housing having defined therein a longitudinal bore; a sensor element inserted into the longitudinal bore; and a sealing arrangement provided in the longitudinal bore, surrounding at least a portion of the sensor element, and comprised of at least one seal element comprised of steatite and an additional seal element comprised of boron nitride arranged in contact with one another and in a stack having a side near the gas to be measured so that, when there is one seal element comprised of steatite, the one seal element comprised of steatite is positioned on the side of the stack near the gas to be measured, and so that, when there are two seal elements comprised of steatite, the additional seal element is positioned between the two seal elements comprised of steatite, wherein the at least one seal element comprised of steatite and the additional seal element comprised of boron nitride are inserted into the longitudinal bore of the housing as deformable rings and are pressed-in therein, and wherein, during the pressing-in, the deformable rings are deformed in such a way that the at least one seal element comprised of steatite and the additional seal element comprised of boron nitride are pushed against the sensor element and the housing.

BACKGROUND OF THE INVENTION

The invention starts with a seal for a sensor element of a gas sensoraccording to the generic type in the Main claim. Such a seal is knownfrom the DE-OS 43 18 789 where the sensor element is arranged in alongitudinal bore of a housing by means of two seal elements and anadditional, deformable seal that is arranged between the seal elements.The two seal elements consist of magnesium-aluminum silicate (steatite).Rough-pressed metal powder or graphite is listed as material for theadditional seal.

SUMMARY OF THE INVENTION

The inventive seal The present invention is a seal for a sensor elementfor a gas sensor, in particular for determining the oxygen content inexhaust gases of internal combustion engines, which seals the sensorelement in a longitudinal bore of a metallic housing, characterized inthat a sealing arrangement is provided, which comprises at least twoseal elements, arranged one above the other, and that the one sealelement is composed of boron nitride and the other seal element ofsteatite has the advantage that it is gas-tight as well as impermeablefor liquids, in particular fuel, and additionally has a high temperatureresistance.

Through the measures listed in the dependent claims, advantageousmodifications of the seal described in the Main claim are possible. Aneasy handling of the seal during assembly is achieved if as sealingrings the steatite seal elements are inserted in the presinteredcondition and the boron nitride seal element is inserted in thehot-pressed condition and if these are deformed during the assemblythrough the effects of a force in such a way that the seal elementmaterial conforms to the sensor element, and the housing and the sensorelement is thus kept gas-tight inside the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

Three embodiments of the invention are shown in the drawing and areexplained in more detail in the following description.

FIG. 1 shows a cross section through a gas sensor with a sealingarrangement according to a first exemplary embodiment,

FIG. 2 the gas sensor with a sealing arrangement according to a secondexemplary embodiment, and

FIG. 3 the gas sensor with a sealing arrangement according to a thirdexemplary embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGS. 1, 2 and 3 show a gas sensor 10, for example anelectrochemical oxygen sensor with a metallic housing 12, which has athread 13 as fastening means for installation in a measuring gas tubethat is not shown. The housing 12 has a longitudinal bore 15 with ashoulder-type ring surface 16. The shoulder-type ring surface holds, forexample, a metallic seal ring 18, on which a molded ceramic part 21rests on the measuring gas side. The molded ceramic part 21 on themeasuring gas side has a continuous opening 22 on the measuring gas sidethat extends in the direction of the longitudinal bore 15.

Furthermore, a molded ceramic part 23 is also arranged on the connectionside in the longitudinal bore 15, at a distance to the molded ceramicpart 21 on the measuring gas side. The molded ceramic part 23 on theconnection side has a centrally located, continuous opening 24 on theconnection side that also extends in the direction of the longitudinalbore 15. The opening 22 on the measuring gas side of the molded ceramicpart 21 on the measuring gas side and the connection-side opening 24 ofthe connection-side molded ceramic part 23 are aligned with each other.A platelike sensor element 27 with an end segment 28 on the measuringgas side and an end segment 29 on the connection side is located in theopenings 22, 24.

The measuring gas side end segment 28 of the sensor element 27 projectsfrom the housing 12 and is surrounded by a protective tube 31, which isattached to the housing 12. The protective tube 31 has intake anddischarge openings 32 for the gas to be measured. The connection sideend segment 29 has connection contacts 34 that also project from thehousing 12. The connection contacts 34 are contacted with a contact plugwith connecting cables, that is not shown. The end segment 29 on theconnection side, which projects from the housing 12, is surrounded by anon-depicted jacket, which protects the end segment 29 fromenvironmental influences.

Between the molded ceramic part 21 on the measuring gas side and themolded ceramic part 23 on the connection side is a sealing arrangement35, comprising a first seal element 36, a second seal element 37 and athird seal element 38. The first seal element 36 is composed of steatiteand rests on the molded ceramic part 21 on the measuring gas side. Thisis joined by the second seal element 37, which is composed of hexagonalboron nitride. Located above the second seal element 37 is the thirdseal element 38, which is also composed of steatite. The molded ceramicpart 23 on the connection side pushes against the third seal element 38.The contact force of the molded ceramic part 23 on the connection sideis generated by a metal sleeve 40. The metal sleeve 40 has, for example,several evenly spaced claws 41 that point toward the inside and engagein notches 42 that are formed into the housing 12. However, it is alsoconceivable that the metal sleeve 40 is welded to the housing 12.

A second exemplary embodiment is shown in FIG. 2. With this exemplaryembodiment, the sealing arrangement 35 consists only of the second sealelement 37 and the third seal element 38. The second seal element 37 ofboron nitride rests directly on the molded ceramic part 21 on themeasuring gas side.

For a third exemplary embodiment according to FIG. 3, the sealingarrangement 35 comprises the first seal element 36 and the second sealelement 37. In this case, the molded ceramic part 23 on the connectionside pushes directly onto the second seal element 37 of boron nitride.

It has proven useful that the volume of seal elements 36, 38, which arecomposed of steatite, is approximately twice the volume of the sealelement 37 of boron nitride. As a result of this, the thickness of theseal elements 36, 37, 38 according to the embodiment in FIG. 1 isapproximately the same. For the two embodiments according to FIGS. 2 and3, on the other hand, the first seal element 36 or the third sealelement 38 is approximately twice as thick in design as the second sealelement 37 of boron nitride.

Prior to installing them in the longitudinal bore 15 of the housing 12,the seal elements 36, 38 of steatite are preformed as rings by sinteringthem at a low temperature of, for example, 500° C. The second sealelement 37 of hexagonal boron nitride, on the other hand, is hot-pressedat approximately 2000° C. The ring-shaped seal elements 36, 37, 38formed in this way are inserted in accordance with the correspondingembodiments into the longitudinal bore 15 that already holds the sensorelement 27. The molded ceramic part 23 on the connection side is thenplaced over the correspondingly designed sealing arrangement 35. Themetal sleeve 40 is then fitted onto the molded ceramic part on theconnection side. Subsequently, a force is exerted onto the metal sleeve40, for example by means of a die, which force acts upon the sealelements 36, 37, 38 of the sealing arrangement 35 via the molded ceramicpart 23 on the connection side. The prefabricated rings of seal elements36, 37, 38 are deformed in such a way that the material for the sealelements 36, 37, 38 presses against the sensor element 27 and thehousing 12.

It appears that the sealing effect is essentially determined by thehexagonal boron nitride of the second seal element 37. The boron nitrideis highly impermeable to gas and fuel as a result of its crystallinestructure.

It is essential for achieving an impermeability to gas and fuel over awide temperature range that a force is present that is constantlyexerted by the metal sleeve 40 onto the seal elements 36, 37, 38. As aresult of the higher thermal coefficient of expansion of steatite(8.8×10⁻⁶ K⁻¹) as compared to boron nitride (approximately 4.4×10⁻⁶K⁻¹), it is achieved that the contact pressure originating with themetal sleeve 40 acts upon the sealing arrangement 35, even at highertemperatures.

The use of the inventive sealing arrangement 35 is not limited to thesealing of planar sensor elements in metallic housings. It is alsoconceivable to use such a sealing arrangement 35 for the sealing ofso-called finger probes. For that type of use, only the design of theprefabricated rings for the seal elements 36, 37, 38 must be adjusted tothe geometry of the longitudinal bore and the supporting surface forhousing and finger-shaped sensor elements.

What is claimed is:
 1. A seal for a sensor element for a gas sensor fordetermining the oxygen content in a gas to be measured including exhaustgases of internal combustion engines, the seal comprising:a metallichousing having defined therein a longitudinal bore; a sensor elementinserted into the longitudinal bore; and a sealing arrangement providedin the longitudinal bore, surrounding at least a portion of the sensorelement, and comprised of at least one seal element comprised ofsteatite and an additional seal element comprised of boron nitridearranged in contact with one another and in a stack having a side nearthe gas to be measured so that ,when there is one seal element comprisedof steatite, the one seal element comprised of steatite is positioned onthe side of the stack near the gas to be measured, and so that, whenthere are two seal elements comprised of steatite, the additional sealelement is positioned between the two seal elements comprised ofsteatite, wherein the at least one seal element comprised of steatiteand the additional seal element comprised of boron nitride are insertedinto the longitudinal bore of the housing as deformable rings and arepressed-in therein, and wherein, during the pressing-in, the deformablerings are deformed in such a way that the at least one seal elementcomprised of steatite and the additional seal element comprised of boronnitride are pushed against the sensor element and the housing.
 2. Theseal according to claim 1, wherein the boron nitride has a crystallinestructure which is hexagonal.
 3. The seal according to claim 1, whereinthe at least one seal element comprised of steatite has a volume,wherein the additional seal element comprised of boron nitride has avolume, and wherein the volume of the at least one seal elementcomprised of steatite is greater than the volume of the additional sealcomprised of boron nitride.
 4. The seal according to claim 3, whereinthe volume of the at least one seal element comprised of steatite isabout twice the volume of the additional seal element comprised of boronnitride.
 5. The seal according to claim 1, wherein the sensor elementhas a side facing the gas to be measured and a connection side, whereinthe seal further comprises a molded ceramic part provided within thelongitudinal bore and on the side facing the gas to be measured and amolded ceramic part provided within the longitudinal bore and on theconnection side and spaced apart from the molded ceramic part providedon the measuring gas side, and wherein the sealing arrangement ispositioned between the molded ceramic parts.
 6. The seal according toclaim 5, further comprising a pressure element provided in the housingand in connection therewith, which pressure element presses on themolded ceramic part provided on the connection side.
 7. The sealingaccording to claim 5, wherein two seal elements comprised of steatiteare provided, wherein the sealing arrangement is positioned on top ofthe molded ceramic part provided on the side of the gas to be measuredso that a stack is provided comprised of one sealing element comprisedof steatite, the additional seal element comprised of boron nitride, andanother sealing element comprised of steatite, and wherein the moldedceramic part provided on the connection side is positioned on top of thestack and exerts pressure thereon.
 8. The seal according to claim 5,wherein the sealing arrangement consists of one seal element comprisingsteatite positioned on top of the molded ceramic part provided on theside of the gas to be measured and the additional seal element comprisedof boron nitride on which is positioned the molded ceramic part providedon the connection side so that pressure is exerted thereby on theadditional seal element.
 9. The seal according to claim 1, whereindeformation of the deformable rings causes conversion of at least aportion of the deformable rings into powder.
 10. The seal according toclaim 1, wherein the deformable ring of the at least one seal elementcomprised of steatite is a sintered preformed ring and the deformablering of the additional seal element comprised of boron nitride is ahot-pressed preformed ring.